I. Field
The present invention relates generally to communication, and more specifically to techniques for controlling the transmit power of a data transmission sent on multiple data channels in a wireless communication system.
II. Background
In a wireless communication system, a wireless device (e.g., a cellular phone) communicates with one or more base stations via transmissions on the downlink and uplink. The downlink (or forward link) refers to the communication link from the base station to the wireless device, and the uplink (or reverse link) refers to the communication link from the wireless device to the base station. In a Code Division Multiple Access (CDMA) system, a base station may transmit data to multiple wireless devices simultaneously. The total transmit power available at the base station thus determines the downlink capacity of the base station. A portion of the total available transmit power is allocated to each active wireless device such that the aggregate transmit power used for all active wireless devices is less than or equal to the total available transmit power.
To maximize downlink capacity, a power control mechanism is typically used for each wireless device. The power control mechanism is normally implemented with two power control loops, which are often called an “inner” loop and an “outer” loop. The inner loop adjusts the transmit power used for the wireless device such that the received signal quality for a downlink data transmission, as measured at the wireless device, is maintained at a target value. The signal quality (denoted as SIR) may be quantified by a signal-to-noise-plus-interference ratio or some other quantity, and the target value may be called an SIR target. The outer loop adjusts the SIR target to achieve the desired level of performance, which may be quantified by a block error rate (BLER) target or some other performance measurement. The wireless device consumes too much system capacity if the BLER is too low and provides unsatisfactory performance if the BLER is too high. By minimizing the amount of transmit power used for the wireless device while maintaining the BLER target, improved utilization of the system capacity and reduced delays in serving wireless users may be achieved.
A Wideband CDMA (W-CDMA) system supports data transmission on one or more “transport” channels to a wireless device. A transport channel may be viewed as a data channel or a data/message bearer. Each transport channel is associated with one or more transport formats, and each transport format specifies various processing parameters for that transport channel. A BLER target may also be specified for each transport channel. Each transport channel may require a different SIR target, which is dependent on various factors such as the BLER target and the transport formats used for that transport channel and the wireless channel condition. Data to be sent on the transport channels are multiplexed onto a “physical” channel. The transmit power for the physical channel (and not the individual transport channels) is adjusted through power control. Power control for a single physical channel carrying multiple transport channels with different SIR targets is challenging, especially if one or more of these transport channels are intermittently active or inactive for an extended period of time.
There is therefore a need in the art for techniques to control the transmit power for multiple data channels power controlled together, where one or more of the data channels may be intermittently active.